Liquid-crystalline medium

ABSTRACT

The invention relates to a liquid-crystalline medium based on a mixture of polar compounds of positive dielectric anisotropy, characterized in that it contains one or more compounds of the general formula I in which Hal is fluorine or chlorine and R is alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each case having up to 7 carbon atoms.

This application is a continuation of application Ser. No. 08/364,033,filed Dec. 27, 1994, now U.S. Pat. No. 5,746,941, which is acontinuation of Ser. No. 07/721,502 filed Jul. 5, 1991.

The present invention relates to a liquid-crystalline medium, to the usethereof for electrooptical purposes, and to displays containing thismedium.

Liquid crystals are used, in particular, as dielectrics in displaydevices since the optical properties of such substances can be affectedby an applied voltage. Electrooptical devices based on liquid crystalsare extremely well known to those skilled in the art and may be based onvarious effects. Devices of this type are, for example, cells havingdynamic scattering, DAP (deformation of aligned phases) cells,guest/host cells, TN cells having a twisted nematic structure, STN(supertwisted nematiic) cells, SBE (super-birefringence effect) cellsand OMI (optical mode interference) cells. The most common displaydevices are based on the Schadt-Helfrich effect and have a twistednematic structure.

The liquid-crystal materials must have good chemical and thermalstability and good stability toward electrical fields andelectromagnetic radiation. Furthermore, the liquid-crystal materialsshould have low viscosity and give short addressing times, low thresholdvoltages and high contrast in the cells. Furthermore, they should have asuitable mesophase, for example, for the abovementioned cells, a nematicor cholesteric meso-phase, at customary operating temperatures, i.e. inthe broadest possible range above and below room temperature.

Since liquid crystals are generally used as mixtures of a plurality ofcomponents, it is important that the components are readily misciblewith one another. Further properties, such as electrical conductivity,dielectric anisotropy and optical anisotropy, must meet variousrequirements depending on the cell type and the area of application. Forexample, materials for cells having a twisted nematic structure shouldhave positive dielectric anisotropy and low electrical conductivity.

For example, the media desired for matrix liquid-crystal displayscontaining integrated nonlinear elements for switching individual imagepoints (MLC displays) are those having high positive dielectricanisotropy, broad nematic phases, relatively low birefringence, veryhigh specific resistance, good UV and temperature stability of theresistance and low vapour pressure.

Matrix liquid-crystal displays of this type are known. Examples ofnonlinear elements which can be used to individually switch theindividual image points are active elements (i.e. transistors). This isthen referred to as an "active matrix", and a differentiation can bemade between two types:

1. MOS (Metal Oxide Semiconductor) transistors on a silicon wafer assubstrate.

2. Thin-film transistors (TFTS) on a glass plate as substrate.

The use of monocrystalline silicon as the substrate material limits thedisplay size since even the modular assembly of various part displaysresults in problems at the joints.

In the case of the more promising type 2, which is preferred, theelectrooptical effect used is usually the TN effect. A differentiationis made between two technologies: TFTs comprising compoundsemiconductors, such as, for example, CdSe, or TFTs based onpolycrystalline or amorphous silicon. Intensive research efforts arebeing made worldwide in the latter technology.

The TFT matrix is applied to the inside of one glass plate of thedisplay, while the inside of the other glass plate carries thetransparent counter-electrode. Compared with the size of the image pointelectrode, the TFT is very small and hardly affects the image at all.This technology can also be extended to fully colour-compatible imagedisplays, where a mosaic of red, green and blue filters is arranged insuch a manner that each filter element is located opposite a switchableimage element.

The TFT displays usually operate as TN cells with crossed polarizers intransmission and are illuminated from the back.

The term MLC displays here covers any matrix display containingintegrated nonlinear elements, i.e. in addition to the active matrix,also displays containing passive elements such as varistors or diodes(MIM=metal-insulator-metal).

MLC displays of this type are particularly suitable for TV applications(for example pocket TV sets) or for high-information displays forcomputer applications (laptops) and in automobile or aircraftconstruction. In addition to problems with respect to the angledependency of the contrast and the switching times, problems result inMLC displays due to inadequate specific resistance of the liquid-crystalmixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E.,SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay84, Sept. 1984: A 210-288, Matrix LCD Controlled by Double Stage DiodeRings, p. 141 ff., Paris; STROMER, M., Proc. Eurodisplay 84, Sept. 1984:

Design of Thin Film Transistors for Matrix Adressing [sic] of TelevisionLiquid Crystal Displays, p. 145 ff., Paris.] As the resistancedecreases, the contrast of an MLC display worsens and the problem of"after image elimination" may occur. Since the specific resistance ofthe liquid-crystal mixture generally decreases over the life of an MLCdisplay due to interaction with the internal surfaces of the display, ahigh (initial) resistance is very important to give acceptable servicelives. In particular, in the case of low-voltage mixtures, it washitherto not possible to achieve very high specific resistances. It isfurthermore important that the specific resistance increases as littleas possible with increasing temperature and after heating and/orexposure to UV radiation. The MLC displays of the prior art do notsatisfy current demands.

Thus, there continues to be a great demand for MLC displays of very highspecific resistance and at the same time a broad operating temperaturerange, short switching times and low threshold voltage which do not havethese disadvantages or only do so to a lesser extent.

For TN (Schadt-Helfrich) cells, media are desired which facilitate thefollowing advantages in the cells:

broadened nematic phase range (in particular down to low temperatures),

switchability at extremely low temperatures (outdoor use, automobiles,avionics),

increased stability to UV radiation (longer life).

The media available from the prior art do not make it possible toachieve these advantages whilst simultaneously retaining the otherparameters.

For supertwisted (STN) cells, media are desired which facilitate agreater multiplexing ability and/or lower threshold voltages and/orbroader nematic phase ranges (in particular at low temperatures). Tothis end, a further extension of the parameter latitude available(clearing point, smectic-nematic transition or melting point, viscosity,dielectric values, elastic values) is urgently desired.

The invention has the object of providing media, in particular for MLC,TN or STN displays of this type, which do not have the abovementioneddisadvantages or only do so to a lesser extent, and preferably at thesame time have very high specific resistances and low thresholdvoltages.

It has now been found that this object can be achieved if mediaaccording to the invention are used in displays. The invention thusrelates to a liquid-crystalline medium based on a mixture of polarcompounds having positive dielectric anisotropy, characterized in thatit contains one or more compounds of the general formula I ##STR2## inwhich Hal is fluorine or chlorine and R is alkyl, oxaalkyl, fluoroalkylor alkenyl, in each case having up to 7 carbon atoms.

The invention also relates to electrooptical displays (in particular STNor MLC displays having two plane-parallel outer plates which, togetherwith a frame, form a cell, integrated nonlinear elements for switchingindividual image points on the outer plates, and a nematicliquid-crystal mixture of positive dielectric anisotropy and highspecific resistance located in the cell) which contain media of thistype, and to the use of these media for electrooptical purposes.

The liquid-crystal mixtures according to the invention facilitate asignificant broadening of the parameter latitude available.

The achievable combinations of clearing point, viscosity at lowtemperature, thermal and UV stability and dielectric anisotropy orthreshold voltage are far superior to previous materials from the priorart.

The requirement for a high clearing point, a nematic phase at -40° C.and a high Δε was previously only achievable to an unsatisfactoryextent. Although systems such as, for example, ZLI-3119 have acomparable clearing point and comparatively favorable viscosities, theyhave, however, a Δε of only +3.

The liquid-crystal mixtures according to the invention make it possibleto achieve, at low viscosities at low temperatures (at -30° C.≦600,preferably ≦550 mPa.S; at -40° C.≦1800, preferably <1700 mPa.s),simultaneously dielectric anisotropy values Δε≧3.5, preferably ≧4.0,clearing points above 65°, preferably above 70°, and a high value forthe specific resistance, which means that excellent STN and MCL displayscan be achieved.

It goes without saying that a suitable choice of the components of themixtures according to the invention also allows higher clearing points(for example above 90°) to be achieved at higher threshold voltages orlower clearing points to be achieved at lower threshold voltages whileretaining the other advantageous properties. The MLC displays accordingto the invention preferably operate in the first transmission minimum ofGooch and Tarry [C. H. Gooch and H. A. Tarry, Electron. Lett. 10, 2-4,1974; C. H. Gooch and H. A. Tarry, Appl. Phys., Vol. 8, 1575-1584,1975], in this case, a lower dielectric anisotropy in the second minimumis sufficient in addition to particularly favorable electroopticalproperties, such as, for example, high gradient of the characteristicline and low angle dependency of the contrast (German Patent 30 22 818)at the same threshold voltage as in an analogous display. This allowssignificantly higher specific resistances to be achieved in the firstminimum using the mixtures according to the invention than usingmixtures containing cyano compounds. A person skilled in the art can usesimple routine methods to produce the birefringence necessary for aprespecified cell thickness of the MLC display by a suitable choice ofthe individual components and their proportions by weight.

The viscosity at 20° C. is preferably ≦25 mPa.s. The nematic phase rangeis preferably at least 70°, in particular at least 80°. This rangepreferably extends at least from -30° to +70°.

Measurements of the "capacity holding ration " (HR) [S. Matsumoto etal., Liquid Crystals 5, 1320 (1989); K. Niwa et al., Proc. SIDConference, San Francisco, June 1984, p. 304 (1984); G. Weber et al.,Liquid Crystals 5, 1381 (1989)] have shown that mixtures according tothe invention containing compounds of the formula I exhibit aconsiderably smaller decrease in the HR with increasing temperature thando analogous mixtures in which the compounds of the formula I arereplaced by cyanophenylcyclohexanes of the formula ##STR3##

The UV stability of the mixtures according to the invention is alsoconsiderably better, i.e. they exhibit a significantly smaller decreasein the HR on exposure to UV radiation.

The media according to the invention are distinguished by extremely highelastic constants at very favorable viscosity values in addition to anunusually broad nematic phase range, resulting, in particular when usedin STN displays, in significant advantages over prior-art media.

The media according to the invention are preferably based on a plurality(preferably two or more) of compounds of the formula I, i.e. theproportion of these compounds is ≧25%, preferably ≧40%.

The individual compounds of the formulae I to XIV and their sub-formulaewhich can be used in the media according to the invention are eitherknown or can be prepared analogously to the known compounds.

Preferred embodiments are indicated below:

a medium additionally contains one or more compounds selected from thegroup comprising the general formulae II, III and IV: ##STR4## in whichthe individual radicals are as defined below: R: alkyl, oxaalkyl,fluoroalkyl or alkenyl, in each case having up to 7 carbon atoms

X: F, Cl, CF₃, OCF₃ or OCHF₂ Y¹ and Y² H or F

r: 0 or 1.

a medium additionally contains one or more compounds selected from thegroup comprising the general formulae V to VIII: ##STR5## in which R, X,Y¹ and Y² are each, independently of one another, as defined as above.

a medium additionally contains one or more compounds selected from thegroup comprising the general formulae IX to XIV: ##STR6## in which R, X,Y¹ and Y² are each, independently of one another, as defined as above.

The proportion of compounds of the formulae I to IV together is at least50% by weight in the total mixture

the proportion of compounds of the formula I is from 10 to 50% by weightin the total mixture

the proportion of compounds of the formulae II to IV is from 30 to 70%by weight in the total mixture ##STR7## the medium contains compounds ofthe formulae II and III or IV R is straight-chain alkyl or alkenylhaving 2 to 7 carbon atoms

the medium essentially comprises compounds of the formulae I to IV

the medium contains further compounds, preferably selected from thefollowing group: ##STR8## The I:(II+III+IV) weight ratio is preferablyfrom 1:4 to 1:1. The medium essentially comprises compounds selectedfrom the group comprising the general formulae I to XIV.

It has been found that even a relatively small proportion of compoundsof the formula I mixed with conventional liquid-crystal materials, butin particular containing one or more compounds of the formula II, IIIand/or IV, results in a significant improvement in the addressing timesand in low threshold voltages, and at the same time broad nematic phaseswith low smectic-nematic transition temperatures are observed. Thecompounds of the formulae I to IV are colorless, stable and readilymiscible with one another and with other liquid-crystal materials.

The term "alkyl" covers straight-chain and branched alkyl groups having1-7 carbon atoms, in particular the straight-chain groups methyl, ethyl,propyl, butyl, pentyl, hexyl and heptyl. Group [sic] having 2-5 carbonatoms are generally preferred.

The term "alkenyl" covers straight-chain and branched alkenyl groupshaving 2-7 carbon atoms, in particular the straight-chain groups.Preferred alkenyl groups are C₂ -C₇ -1E-alkenyl, C₄ -C₇ -3E-alkenyl, C₅-C₇ -4-alkenyl, C₆ -C₇ -5-alkenyl and C₇ -6-alkenyl, in particular C₂-C₇ -1E-alkenyl, C₄ -C₇ -3E-alkenyl and C₅ -C₇ -4-alkenyl. Examples ofpreferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl,1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl,3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl,4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5carbon atoms are generally preferred.

The term "fluoroalkyl" preferably covers straight-chain groupscontaining terminal fluorine, i.e. fluoromethyl, 2-fluoroethyl,3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and7-fluoroheptyl. However, other positions of the fluorine are notexcluded.

The term "oxaalkyl" preferably covers straight-chain radicals of theformula C_(n) H_(2n+1) --O--(CH₂)_(m), in which n and m are each,independently of one another, from 1 to 6. n is preferably 1 and m ispreferably from 1 to 6.

Through a suitable choice of the meanings of R, X and Y, the addressingtimes, the threshold voltage, the gradient of the transmissioncharacteristic lines, etc., can be modified as desired. For example,1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxy radicals and thelike generally give shorter addressing times, improved nematictendencies and a higher ratio between the elastic constants k₃₃ (bend)and k₁₁ (splay) compared with alkyl and alkoxy radicals. 4-Alkenylradicals, 3-alkenyl radicals and the like generally give lower thresholdvoltages and lower values of k₃₃ /k₁₁ compared with alkyl and alkoxyradicals. A --CH₂ CH₂ -- group in Z¹ or Z² generally results in highervalues of k₃₃ /k₁₁ compared with a simple covalent bond. Higher valuesof k₃₃ /k₁₁ facilitate, for example, flatter transmission characteristiclines in TN cells with a 90° twist (for achieving gray tones) andsteeper transmission characteristic lines in STN, SBE and OMI cells(greater multiplexing ability), and vice versa.

The optimum mixing ratios of the compounds of the formulae I andII+III+IV depends substantially on the desired properties, on the choiceof the components of the formulae I, II, III and/or IV and on the choiceof any other components which may be present. Suitable mixing ratioswithin the abovementioned range can easily be determined from case tocase.

The total amount of compounds of the formulae I to XIV in the mixturesaccording to the invention is not crucial. The mixtures may thereforecontain one or more further components in order to optimize variousproperties. However, the effect observed on the addressing times and thethreshold voltage is generally greater the higher the totalconcentration of compounds of the formulae I to XIV.

In a particularly preferred embodiment, the media according to theinvention contain compounds of the formula II, III, V and/or VII(preferably II and/or III) in which X is CF₃, OCF₃ or OCHF₂. A favorablesynergistic effect with the compounds of the formula I results inparticularly advantageous properties.

For STN applications, the media preferably contain compounds selectedfrom the group comprising the formulae V to VIII in which X ispreferably OCHF₂.

The media according to the invention may furthermore contain a componentA comprising one or more compounds of the general formula I' having adielectric anisotropy of from -1.5 to +1.5 ##STR9## in which R¹ and R²are each, independently of one another, n-alkyl, n-alkoxy, ω-fluoroalkylor n-alkenyl having up to 9 carbon atoms,

the rings A¹, A² and A³ are each, independently of one another,1,4-phenylene, 2- or 3-fluoro-1,4-phenylene, trans-1,4-cyclohexylene or1,4-cyclohexenylene,

Z¹ and Z² are each, independently of one another, --CH₂ CH₂ --,--C.tbd.E--, --CO--O--, --O--CO-- or a single bond,

and

m is 0, 1 or 2.

Component A preferably contains one or more compounds selected from thegroup comprising II1 to II7: ##STR10## in which R¹ and R² are as definedunder formula I'.

Component A preferably additionally contains one or more compoundsselected from the group comprising II8 to II20: ##STR11## in which R¹and R² are as defined under formula I', and the 1,4-phenylene groups inII8 to II17 may each, independently of one another, also bemonosubstituted or polysubstituted by fluorine.

Furthermore, component A preferably additionally contains one or morecompounds selected from the group comprising II21 to II25 contains:##STR12## in which R¹ and R² are as defined under formula I', and the1,4-phenylene groups in II21 to II25 may also each, independently of oneanother, be monosubstituted or polysubstituted by fluorine.

Finally, preferred mixtures of this type are those in which component Acontains one or more compounds selected from the group comprising II26and II27: ##STR13## in which C_(r) H_(2r+1) is a straight-chain alkylgroup having up to 7 carbon atoms.

In some cases, the addition of compounds of the formula ##STR14## inwhich R¹ and R² are as defined under formula I'

and

Z⁰ is a single bond, --CH₂ CH₂ --, ##STR15## or proves advantageous forsuppressing smectic phases, although this may reduce the specificresistance. In order to achieve parameter combinations which are idealfor the application, a person skilled in the art can easily determinewhether and, if yes, in what amount these compounds may be added.Normally, less than 15%, in particular 5-10%, are used.

Preference is also given to liquid-crystal mixtures which contain one ormore compounds selected from the group comprising III' and IV':##STR16## in which R¹ and R² are as defined under formula I'.

The type and amount of the polar compounds having positive dielectricanisotropy is not crucial per se. A person skilled in the art can usesimple routine experiments to select suitable materials from a widerange of known and, in many cases, also commercially availablecomponents and base mixtures. The media according to the inventionpreferably contain one or more compounds of the formula II" ##STR17## inwhich Z¹, Z² and m are as defined under the formula I', Q¹ and Q² areeach, independently of one another, 1,4-phenylene,trans-1,4-cyclohexylene or 3-fluoro-1,4-phenylene-, and one of theradicals Q¹ and Q² is alternatively trans-1,3-dioxane-2,5-diyl,pyrimidine-2,5-diyl, pyridine-2,5-diyl or 1,4-cyclohexenylene,

R⁰ is n-alkyl, n-alkenyl, n-alkoxy or n-oxaalkyl, in each case having upto 9 carbon atoms, Y is H or F and X' is CN, halogen, CF₃, OCF₃ orOCHF₂.

In a preferred embodiment, the media according to the invention for STNor TN applications are based on compounds of the formula I" in which X'is CN. It goes without saying that smaller or larger proportions ofother compounds of the formula I" (X'≠CN) are also possible. For MLCapplications, the media according to the invention preferably containonly up to about 10% of nitrites of the formula I" (but preferably nonitrites of the formula I", but instead compounds of the formula I'where X'=halogen, CF₃, OCF₃ or OCHF₂). These media are preferably basedon the compounds of the formulae II to XIV.

The construction of the STN and MLC displays according to the inventionfrom polarizers, electrode base plates and electrodes with surfacetreatment corresponds to the construction which is conventional fordisplays of this type. The term conventional construction here is widelydrawn and also covers all derivatives and modifications of the MLCdisplay, in particular also matrix display elements based on poly-SiTFTs or MIMs.

An essential difference between the displays according to the inventionand those customary hitherto based on the twisted nematic cell is,however, the choice of liquid-crystal parameters in the liquid-crystallayer.

The liquid-crystal mixtures which can be used according to the inventionare prepared in a manner which is conventional per se. In general, thedesired amount of the components used in the lesser amount is dissolvedin the components making up the principal constituent, expediently atelevated temperature. It is also possible to mix solutions of thecomponents in an organic solvent, for example in acetone, chloroform ormethanol, and, after thorough mixing, to remove the solvent again, forexample by distillation.

The dielectrics may also contain other additives known to those skilledin the art and described in the literature. For example, 0-15% ofpleochroic dyes or chiral dopes can be added.

C denotes a crystalline phase, S a smectic phase, S_(B) a smectic Bphase, N a nematic phase and I the isotropic phase.

V₁₀ denotes the voltage for 10% transmission (view angle perpendicularto the plate surface). t_(on) denotes the switch-on time and t_(off) theswitch-off time at an operating voltage corresponding to 2.5 times thevalue of V₁₀ •n [sic] denotes the optical anisotropy and n. therefractive index. AE denotes the dielectric anisotropy (Δε=ε.sub.∥-ε.sub.⊥, where ε.sub.∥ is the dielectric constant parallel to thelongitudinal molecular axes and ε.sub.⊥ is the dielectric constantperpendicular thereto. The electrooptical data were measured in a TNcell at the 1st minimum (i.e. at a d•Δn value of 0.5) at 20° C., unlessexpressly stated otherwise. The optical data were measured at 20° C.,unless expressly stated otherwise.

The examples below are intended to illustrate the invention withoutrepresenting a limitation. Above and below all temperatures are given in° C. The percentages are percent by weight.

In the present application and in the examples below, the structures ofthe liquid-crystal compounds are indicated by acronyms, with thetransformation into chemical formulae taking place in accordance withTables A and B below. All radicals C_(n) H_(2n+1) are straight-chainalkyl radicals containing n or m carbon atoms. The coding in Table B isself-evident. In Table A, only the acronym for the base structure isgiven. In individual cases, the acronym for the base structure isfollowed, separated by a hyphen, by a code for the substituents R¹, R²,L¹, L² and L³ :

    __________________________________________________________________________    Code for R.sup.1,                                                             R.sup.2, L.sup.1, L.sup.2                                                            R.sup.1       R.sup.2 L.sup.1                                                                          L.sup.2                                                                          L.sup.3                                    __________________________________________________________________________    nm     C.sub.n H.sub.2n+1                                                                          C.sub.m H.sub.2m+1                                                                    H  H  H                                          nOm    C.sub.n H.sub.2n+1                                                                          OC.sub.m H.sub.2m+1                                                                   H  H  H                                          nO.m   OC.sub.n H.sub.2n+1                                                                         C.sub.m H.sub.2m+1                                                                    H  H  H                                          n      C.sub.n H.sub.2n+1                                                                          CN      H  H  H                                          nN.F   C.sub.n H.sub.2n+1                                                                          CN      H  F  H                                          nF     C.sub.n H.sub.2n+1                                                                          F       H  H  H                                          nOF    OC.sub.n H.sub.2n+1                                                                         F       H  H  H                                          nCl    C.sub.n H.sub.2n+1                                                                          Cl      H  H  H                                          nF.F   C.sub.n H.sub.2n+1                                                                          F       H  F  H                                          nOmFF  C.sub.n H.sub.2n+1                                                                          OC.sub.m H.sub.2m+1                                                                   F  F  H                                          nmF    C.sub.n H.sub.2n+1                                                                          C.sub.m H.sub.2m+1                                                                    F  H  H                                          nCF.sub.3                                                                            C.sub.n H.sub.2n+1                                                                          CF.sub.3                                                                              H  H  H                                          nOCF.sub.3                                                                           C.sub.n H.sub.2n+1                                                                          OCF.sub.3                                                                             H  H  H                                          nOCF.sub.2                                                                           C.sub.n H.sub.2n+1                                                                          OCHF.sub.2                                                                            H  H  H                                          nS     C.sub.n H.sub.2n+1                                                                          NCS     H  H  H                                          rVsN   C.sub.r H.sub.2r+1 --CH═CH--C.sub.s H.sub.2s --                                         CN      H  H  H                                          rEsN   C.sub.r H.sub.2r+1 --O--C.sub.s H.sub.2s --                                                 CN      H  H  H                                          nNF    C.sub.n H.sub.2n+1                                                                          CN      F  H  H                                          nAm    C.sub.n H.sub.2n+1                                                                          COOC.sub.m H.sub.2m+1                                                                 H  H  H                                          nF.F.F C.sub.n H.sub.2n+1                                                                          F       H  F  F                                          nCl.F.F                                                                              C.sub.n H.sub.2n+1                                                                          Cl      H  F  F                                          nCF.sub.3.F.F                                                                        C.sub.n H.sub.2n+1                                                                          CF.sub.3                                                                              H  F  F                                          nOCF.sub.3.F.F                                                                       C.sub.n H.sub.2n+1                                                                          OCF.sub.3                                                                             H  F  F                                          nOCF.sub.2.F.F                                                                       C.sub.n H.sub.2n+1                                                                          OCHF.sub.2                                                                            H  F  F                                          nOCF.sub.3.F                                                                         C.sub.n H.sub.2n+1                                                                          OCF.sub.3                                                                             H  F  H                                          __________________________________________________________________________

                  TABLE A                                                         ______________________________________                                        1 #STR18##                                                                    7 #STR19##                                                                    2 #STR20##                                                                    8 #STR21##                                                                    3 #STR22##                                                                    9 #STR23##                                                                    4 #STR24##                                                                    0 #STR25##                                                                    5 #STR26##                                                                    1 #STR27##                                                                    6 #STR28##                                                                    2 #STR29##                                                                    3 #STR30##                                                                    9 #STR31##                                                                    4 #STR32##                                                                    0 #STR33##                                                                    5 #STR34##                                                                    1 #STR35##                                                                    6 #STR36##                                                                    2 #STR37##                                                                    7 #STR38##                                                                    3 #STR39##                                                                    8 #STR40##                                                                    4 #STR41##                                                                    5 #STR42##                                                                    7 #STR43##                                                                    6 #STR44##                                                                    8 #STR45##                                                                    ______________________________________                                    

                  TABLE B                                                         ______________________________________                                        9 #STR46##                                                                    1 #STR47##                                                                    0 #STR48##                                                                    2 #STR49##                                                                    3 #STR50##                                                                    4 #STR51##                                                                    9 #STR52##                                                                    5 #STR53##                                                                    6 #STR54##                                                                    7 #STR55##                                                                    8 #STR56##                                                                    0 #STR57##                                                                    1 #STR58##                                                                    2 #STR59##                                                                    3 #STR60##                                                                    4 #STR61##                                                                    7 #STR62##                                                                    5 #STR63##                                                                    6 #STR64##                                                                    8 #STR65##                                                                    9 #STR66##                                                                    0 #STR67##                                                                    1 #STR68##                                                                    2 #STR69##                                                                    3 #STR70##                                                                    ______________________________________                                    

    ______________________________________                                        Example 1                                                                     PCH-3        8%                                                               PCH-3Cl.F    12%                                                              PCH-5Cl.F    10%                                                              CCP-20CF.sub.3                                                                             5%                                                               CCP-30CF.sub.3                                                                             13%                                                              CCP-40CF.sub.3                                                                             5%                                                               CCP-50CF.sub.3                                                                             12%                                                              BCH-32       8%                                                               BCH-52       8%                                                               CCPC-33      4%                                                               CCPC-34      4%                                                               CCPC-35      4%                                                               PCH-5F       7%                                                               Example 2                                                                     PCH-3Cl.F    11%          T.sub.c = 71° C.                             PCH-5Cl.F    10%          Δn = 0.09                                     PCH-7F       10%          η = 18 mPa.s                                    CCP-20CF.sub.3                                                                             9%           V.sub.10 = 1.6 V                                    CCP-30CF.sub.3                                                                             12%                                                              CCP-40CF.sub.3                                                                             7%                                                               CCP-50CF.sub.3                                                                             12%                                                              BCH-3F.F     10%                                                              BCH-5F.F     8%                                                               BCH-3.FCF.sub.3                                                                            5%                                                               CCB-3.FF     3%                                                               CCB-5.FF     3%                                                               Example 3                                                                     PCH-3Cl.F    12%                                                              PCH-5Cl.F    10%                                                              EPCH-3F.F    10%                                                              EPCH-5F.F    10%                                                              CCP-3F.F     15%                                                              CCP-5F.F     12%                                                              BCH-3F.F     12%                                                              BCH-30CF.sub.3                                                                             8%                                                               CCB-3.FF     3%                                                               CCB-5F.F     3%                                                               CBC-33F      3%                                                               CBC-55F      2%                                                               Example 4                                                                     PCH-7F.F     10%                                                              PCH-5F.F     12%                                                              B-3F.F       6%                                                               B-5F.F       6%                                                               CCP-2F.F     10%                                                              CCP-3F.F     12%                                                              CCP-5F.F     12%                                                              BCH-3F.F     10%                                                              BCH-5F.F     10%                                                              CCEB-3F.F    3%                                                               CCEB-5F.F    3%                                                               CBC-33F      3%                                                               CBC-53F      3%                                                               Example 5                                                                     PCH-3Cl      15%                                                              PCH-5Cl      10%                                                              PCH-3Cl.F    10%                                                              BECH-3F.F    10%                                                              CCP-2F.F     10%                                                              CCP-3F.F     12%                                                              CCP-5F.F     12%                                                              CCB-3.FF     3%                                                               CCB-5.FF     3%                                                               Example 6                                                                     PCH-3Cl      12%                                                              PCH-5Cl      10%                                                              PCH-3Cl.F    10%                                                              BECH-3F.F    8%                                                               CCP-2F.F     8%                                                               CCP-3F.F     12%                                                              CCP-5F.F     12%                                                              CCB-3.FF     3%                                                               CCB-5.FF     3%                                                               BCH-3F.F     12%                                                              BCH-5F.F     10%                                                              Example 7                                                                     PCH-5Cl      12%                                                              PCH-3Cl      10%          T.sub.c = 91° C.                             PCH-3Cl.F    10%          Δn = 0.09                                     CCP-3Cl.F    13%          Δε = +5                               ECCP-3F.F    13%          V.sub.10 = 2.2 V                                    CBC-53F      4%                                                               CBC-33F      3%                                                               CBC-55F      3%                                                               CCP-50CF.sub.3                                                                             12%                                                              ECCP-30CF.sub.3                                                                            11%                                                              ECCP-50CF.sub.3                                                                            9%                                                               Example 8                                                                     PCH-5Cl      10%                                                              PCH-3Cl.F    8%                                                               PCH-5Cl.F    8%                                                               CCP-20CF.sub.2.F                                                                           10%                                                              CCP-30CF.sub.2.F                                                                           12%                                                              CCP-50CF.sub.2.F                                                                           12%                                                              BCH-3.FF     6%                                                               BCH-3.FCF.sub.3                                                                            8%                                                               BCH-3F.F     10%                                                              CCB-2F.F     3%                                                               CCB-3F.F     3%                                                               CBC-33F      3%                                                               CBC-53F      4%                                                               CBC-55F      3%                                                               Example 9                                                                     PCH-3        9%                                                               PCH-30CF.sub.2                                                                             12%                                                              PCH-50CF.sub.2                                                                             10%                                                              PCH-3Cl.F    8%                                                               ECCP-32      7%                                                               ECCP-33      7%                                                               BCH-3F.F     15%                                                              CCPC-33      3%                                                               CCPC-34      3%                                                               CCPC-35      3%                                                               ECCP-3F.F    13%                                                              ECCP-5F.F    10%                                                              Example 10                                                                    PCH-7Cl      10%                                                              PCH-5Cl      10%                                                              PCH-5Cl.F    8%                                                               PCH-3OCF.sub.3                                                                             6%                                                               PCH-5OCF.sub.3                                                                             6%                                                               CCP-2Cl      8%                                                               CCP-3Cl      8%                                                               CCP-5F       12%                                                              BCH-3F.F     15%                                                              BCH-5F.F     7%                                                               CCB-3F.F     3%                                                               CCB-5F.F     3%                                                               CBC-33F      4%                                                               Example 11                                                                    PCH-5F.F     6%           T.sub.c = 82° C.                             PCH-6F.F     3%           η = 17 mPa.s                                    PCH-7F.F     6%           V.sub.th = 1.8 V                                    CCP-3F.F     12%          Δn = 0.089                                    CCP-5F.F     11%                                                              CECP-2F.F    14%                                                              CECP-3F.F    12%                                                              CECP-5F.F    11%                                                              BCH-3F.F     14%                                                              BCH-5F.F     11%                                                              Example 12                                                                    PCH-5F.F     5%           T.sub.c = 78° C.                             PCH-7F.F     7%           η = 17 mPa.s                                    CCP-3F.F     10%          V.sub.10 = 1.8 V                                    CCP-5F.F     8%           Δn = 0.085                                    CECP-2F.F    10%                                                              CECP-3F.F    12%                                                              CECP-5F.F    9%                                                               CCP-3F.F.F   10%                                                              CCP-5F.F.F   10%                                                              BCH-4F.F.F   10%                                                              BCH-5F.F.F   9%                                                               Example 13                                                                    PCH-5F.F     6%           T.sub.c = 88° C.                             CCP-3F.F     10%          η = 19 mPa.s                                    CCP-5F.F     8%           V.sub.10 = 1.9 V                                    CECP-2F.F    12%          Δn = 0.089                                    CECP-3F.F    14%                                                              CECP-5F.F    11%                                                              CCP-3F.F.F   10%                                                              CCP-5F.F.F   10%                                                              BCH-3F.F.F   10%                                                              BCH-5F.F.F   9%                                                               Example 14                                                                    PCH-5F.F     6%           T.sub.c = 89° C.                             PCH-7F.F     6%           η = 20 mPa.s                                    CCP-3OCF.sub.3                                                                             10%          V.sub.10 = 1.85 V                                   CCP-5OCF.sub.3                                                                             8%           Δn = 0.087                                    ECCP-2F.F    10%                                                              ECCP-3F.F    12%                                                              ECCP-5F.F    9%                                                               CCP-3CF.sub.3.F                                                                            10%                                                              CCP-5CF.sub.3.F                                                                            10%                                                              BCH-3F.F.F   10%                                                              BCH-5F.F.F   9%                                                               Example 15                                                                    PCH-5F.F     6%           T.sub.c = 87° C.                             PCH-7F.F     6%           η = 20 mPa.s                                    CCP-3OCF.sub.3                                                                             10%          V.sub.10 = 1.8 V                                    CCP-5OCF.sub.3                                                                             8%           Δn = 0.093                                    ECCP-3F.F    12%                                                              ECCP-5F.F    9%                                                               CCP-3CF.sub.3.F                                                                            10%                                                              CCP-5CF.sub.3.F                                                                            10%                                                              BCH-3F.F.F   10%                                                              BCH-4F.F.F   10%                                                              BCH-5F.F.F   9%                                                               Example 16                                                                    PCH-5F.F     6%           T.sub.c = 85° C.                             PCH-7F.F     6%           η = 18 mPa.s                                    ECCP-2F.F    12%.         V.sub.10 = 1.9 V                                    ECCP-3F.F    13%          Δn = 0.093                                    ECCP-5F.F    9%                                                               CCP-3F.F.F   11%                                                              CCP-5F.F.F   10%                                                              BCH-2F.F     12%                                                              BCH-3F.F     12%                                                              BCH-5F.F     9%                                                               Example 17                                                                    PCH-5F.F     10%          T.sub.c = 81° C.                             ECCP-3F.F    12%          η = 18 mPa.s                                    ECCP-5F.F    10%          V.sub.10 = 1.9 V                                    CCP-2F.F.F   8%           Δn = 0.092                                    CCP-3F.F.F   10%                                                              CCP-4F.F.F   8%                                                               CCP-5F.F.F   9%                                                               BCH-2F.F     12%                                                              BCH-3F.F     12%                                                              BCH-5F.F     9%                                                               Example 18                                                                    PCH-5Cl.F    6%           T.sub.c = 83° C.                             PCH-6Cl.F    3%           η = 19 mPa.s                                    PCH-7Cl.F    6%           V.sub.th = 1.85 V                                   CCP-3F.F     12%          Δn = 0.093                                    CCP-5F.F     11%                                                              CECP-2F.F    14%                                                              CECP-3F.F    12%                                                              ECCP-5F.F    11%                                                              BCH-3F.F     14%                                                              BCH-5F.F     11%                                                              Example 19                                                                    PCH-5Cl.F    5%           T.sub.c = 81° C.                             PCH-7Cl.F    7%           η = 20 mPa.s                                    CCP-3F.F     10%          V.sub.10 = 1.9 V                                    CCP-5F.F     8%           Δn = 0.092                                    CECP-2F.F    10%                                                              CECP-3F.F    12%                                                              CECP-5F.F    9%                                                               CCP-3F.F.F   10%                                                              CCP-5F.F.F   10%                                                              BCH-3F.F.F   10%                                                              BCH-5F.F.F   9%                                                               Example 20                                                                    PCH-5Cl.F    6%           T.sub.c = 88° C.                             PCH-7Cl.F    6%           η = 21 mPa.s                                    ECCP-2F.F    12%          V.sub.10 = 2.0 V                                    ECCP-3F.F    13%          Δn = 0.099                                    ECCP-5F.F    9%                                                               CCP-3F.F.F   11%                                                              CCP-5F.F.F   10%                                                              BCH-2F.F     12%                                                              BCH-3F.F     12%                                                              BCH-5F.F     9%                                                               Example 21                                                                    PCH-5Cl.F    5%           T.sub.c = 76° C.                             PCH-7Cl.F    6%           V.sub.10 = 1.58 V                                   CCP-2CF.sub.3.F.F                                                                          8%           Δn = 0.098                                    CCP-3CF.sub.3.F.F                                                                          10%                                                              CCP-5CF.sub.3.F.F                                                                          8%                                                               CCP-2Cl.F.F  14%                                                              CCP-3Cl.F.F  12%                                                              CCP-5Cl.F.F  12%                                                              BCH-3F.F     15%                                                              BCH-5F.F     10%                                                              Example 22                                                                    PCH-5F.F     5%           T.sub.c = 66° C.                             PCH-6F.F     3%           η = 21 mPa.s                                    PCH-7F.F     4%           Δn = 0.090                                    CCP-20CF.sub.3.F                                                                           9%           V.sub.10 = 1.4 V                                    CCP-3OCF.sub.3.F                                                                           12%                                                              CCP-4OCF.sub.3.F                                                                           10%                                                              CCP-5OCF.sub.3.F                                                                           12%                                                              CUP-2F       6%                                                               CUP-2F       9%                                                               CUP-5F       11%                                                              CEUP-3F      10%                                                              CEUP-5F      9%                                                               Example 23                                                                    PCH-5F.F     5%           T.sub.c = 95° C.                             PCH-7F.F     6%           Δn = 0.090                                    CCP-2OCF.sub.3.F                                                                           7%           V.sub.10 = 1.85 V                                   CCP-3OCF.sub.3.F                                                                           10%                                                              CCP-5OCF.sub.3.F                                                                           9%                                                               CUP-3F       12%                                                              CUP-5F       12%                                                              CEUP-3F      11%                                                              CEUP-5F      12%                                                              CCB-3.FOCF.sub.3                                                                           6%                                                               CCB-4.FOCF.sub.3                                                                           5%                                                               CCB-5.FOCF.sub.3                                                                           5%                                                               Example 24                                                                    PCH-5Cl.F    5%           T.sub.c = 97° C.                             PCH-7Cl.F    6%           Δn = 0.099                                    CCP-2OCF.sub.3.F                                                                           7%           V.sub.10 = 1.8 V                                    CCP-5OCF.sub.3.F                                                                           10%                                                              CCP-5OCF.sub.3.F                                                                           9%                                                               CUP-3F       12%                                                              CUP-5F       12%                                                              CEUP-3F      11%                                                              CEUP-5F      12%                                                              CCB-3.FOCF.sub.3                                                                           6%                                                               CCB-4.FOCF.sub.3                                                                           5%                                                               CCB-5.FOCF.sub.3                                                                           5%                                                               Example 25                                                                    PCH-5F.F     8%           T.sub.c = 80° C.                             PCH-7F.F     8%           η = 19 mPa.s                                    CCP-2F.F     8%           Δn = 0.078                                    CCP-3F.F     12%          V.sub.10 = 1.55 V                                   CCP-5F.F     12%                                                              CECP-3F.F    10%                                                              CECP-5F.F    9%                                                               ECCP-2F.F    12%                                                              ECCP-3F.F    12%                                                              ECCP-5F.F    9%                                                               Example 26                                                                    PCH5F.F      7%           T.sub.c = 83° C.                             PCH5F.F      6%           η = 17 mPa.s                                    CCP-2OCF.sub.3.F                                                                           11%          Δn = 0.093                                    CCP-3OCF.sub.3.F                                                                           12%          V.sub.10 = 1.65 V                                   CCP-4OCF.sub.3.F                                                                           9%                                                               CCP-5OCF.sub.3.F                                                                           11%                                                              ECCP-2F.F    9%                                                               ECCP-3F.F    8%                                                               ECCP-5F.F    9%                                                               BCH-3CF.sub.3.F.F                                                                          12%                                                              BCH-5CF.sub.3.F.F                                                                          9%                                                               ______________________________________                                    

We claim:
 1. Liquid-crystalline medium based on a mixture of polarcompounds having positive dielectric anisotropy, characterized in thatit contains one or more compounds of the general formula I ##STR71## inwhich Hal is fluorine or chlorine and R is alkyl, fluoroalkyl oralkenyl, in each case having up to 7 carbon atoms, and additionallycontains one or more compounds of one of the general formulae II, III,and IV: ##STR72## in which the individual radicals are as defined below:R* is alkenyl, in each case having up to 7 carbon atoms;X, is F, Cl,CF₃, OCF₃ or OCHF₂ ; Y¹ and Y² are in each case independently of eachother H or F; and r is 0 or
 1. 2. A medium according to claim 1,characterized in that it additionally contains one or more compounds ofone of the general formulae V to VIII: ##STR73## in which R, r, X, Y¹and Y² are each, independently of one another, as defined in claim
 1. 3.A medium according to claim 1, characterized in that it additionallycontains one or more compounds selected from the group consisting ofcompounds of the general formulae IX to XIV: ##STR74## in which R, X, Y¹and Y² are each, independently of one another, as defined in claim
 1. 4.A medium according to claim 2, characterized in that it additionallycontains one or more compounds selected from the group consisting ofcompounds of the general formulae IX to XIV: ##STR75## in which R, X, Y¹and Y² are each, independently of one another, as defined in claim
 2. 5.A medium according to claim 1, characterized in that the proportion ofcompounds of the formulae I to IV together is at least 50% by weight,based on the weight of the total mixture.
 6. A medium according to claim1, characterized in that the proportion of compounds of the formula I isfrom 10 to 50% by weight, based on the weight of the total mixture.
 7. Amedium according to claim 1, characterized in that the proportion ofcompounds of the formulae II to IV is from 30 to 70% by weight, based onthe weight of the total mixture.
 8. A medium according to claim 2,characterized in that the proportion of compounds of the formulae II toIV is from 30 to 70% by weight, based on the weight of the totalmixture.
 9. A medium according to claim 3, characterized in that theproportion of compounds of the formulae II to IV is from 30 to 70% byweight, based on the weight of the total mixture.
 10. A medium accordingto claim 3, characterized in that it consists essentially of compoundsselected form the group consisting of compounds of the general formulaeI to XIV.
 11. A method of using a liquid-crystalline medium according toclaim 1, which comprises incorporating said liquid-crystalline medium inelectrooptical devices.
 12. An electrooptical liquid-crystal displaycontaining a liquid-crystalline medium according to claim 1.